1. Field of the Invention
The present invention relates generally to tape transport systems, and more specifically to a system and method for determining the velocity of a tape in a capstanless tape transport system.
2. Related Art
Magnetic tape has long been used as a mechanism to store data for access by computers. Magnetic tape currently provides low cost access to large quantities of data in a non-volatile form. The magnetic tape is made from a thin polyester layer coated with a magnetic or ferromagnetic material.
Basically, most tape transport systems are two-reel systems. There is a supply reel and a take-up reel. The supply reel supplies the tape to the transport system. The take-up reel is the reel upon which the tape is wound as it is pulled off of the supply reel and read from or written to by the transport system. In reel-to-reel transports, the supply reel is removable so that numerous tapes can be mounted on the tape drive.
Start-stop tape transports use a capstan to help overcome the inertia of the reel mass. With the capstan tape velocity can be accurately controlled. However, over time, the coefficient of friction of the capstan can decrease making tape slippage during acceleration more likely. Such slippage may cause erroneous recording of data. U.S. Pat. No. 3,805,286, to Bunker, describes one way in which such slippage can be detected.
Streaming tape transports, do not use a capstan to help control tape velocity. In this system, the tape is suspended between the two reels and, consequently, is directly controlled thereby. Systems such as this that do not use a capstan to control the speed are subject to error due to variations in the media velocity. To maintain accuracy, complex equations are used to determine velocity by computing ratios of tachometers attached to both reels. State of the art processors, high resolution tachometers, and complex algorithms do an excellent job at monitoring velocity.
The present invention is directed toward an apparatus and method for detecting the velocity of a tape in a tape transport system. The tape transport system has a read head and a write head, separated from each other by a known distance. A write formatter writes data to the tape and formats the data by writing marks such as sync marks and burst marks. For each mark written, the write formatter generates a write signal which is preferably a pulse. Preferably, the pulse is generated at substantially the same time the mark is written to the tape.
The recorded data and marks are sensed by the read head. A detector detects the presence of the marks and generates a read signal or pulse indicating that a mark was read.
An accumulator comprises a plurality of accumulator stages, which can be implemented using timers. Each time the accumulator receives a write pulse, an accumulator stage is started. The stage counts, or otherwise keeps track of the amount of time elapsed since receipt of the write signal and saves this time as a count value. When a read signal is detected, a velocity detector reads the count value stored in a timer stage and determines the velocity of the tape. The velocity is determined as a function of the time it took for a given mark to travel the known distance between the read and write heads.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.